Ukraine's new president declared Sunday a day of mourning and vowed to punish those responsible after pro-Russia separatists shot down a Ukrainian military transport plane Saturday, killing all 49 crew and troops aboard.

It was a bitter setback for the Ukrainian forces - the deadliest single incident yet in their escalating battle against an armed insurgency that the government, backed by the U.S., insists is supported by Russia.

The downing of the plane drew condemnation and concern from the White House, European leaders and U.N. Secretary-General Ban Ki-moon.

This "will refocus attention on the fact that Russia does not seem to be doing very much to moderate the insurgency," said Timothy Ash, an analyst at Standard Bank PLC. "I would expect the focus to return to sanctions next week."

Ukrainian President Petro Poroshenko vowed to punish "those responsible for the tragedy in Luhansk."

In a conversation with French President Francois Hollande, Poroshenko expressed hope that the European Union would decide on further sanctions against Russia if what he called the illegal border crossings and the supply of weapons did not cease.

Nine crew and 40 troops were aboard the Il-76 troop transport when it went down early Saturday as it approached the airport at Luhansk, the Ukrainian prosecutor general's office said.

Luhansk, a city near the border with Russia, is one of two eastern areas where separatists have seized government buildings and declared independence. Ukrainian forces still control the Luhansk airport.

Tensions between Ukraine and Russia escalated in February after pro-Russian President Viktor Yanukovych was driven from office by protesters who wanted closer ties with the European Union and an end to the country's endemic corruption. Russia then seized and annexed Ukraine's Black Sea peninsula of Crimea.